Electro-thermal readout of coulometers

ABSTRACT

Systems are disclosed for reading the integral stored in a coulometer by sensing the thermal properties of the electrolyte. A preferred embodiment comprises at least one temperature sensitive element and a heat source placed adjacent the coulometer positioned in such a manner with respect to each other that the heat conduction path extends through a portion of the coulometer. The thermal conductivity of the path is changed by the migration of the electrolyte into that portion of the coulometer that includes the conduction path.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Application Ser. No.538,464, entitled "Electro-Thermal Readout Coulometer," filed Jan. 3,1975.

BACKGROUND OF THE INVENTION

The present invention relates to electrochemical devices known ascoulometers and more specifically to coulometer-type instrument which iscapable of measuring and indicating the total electrical current thathas been conducted through an electrical circuit.

Coulometers are described in detail in Lester Corrsin's U. S. ReissuePat. No. Re. 27,556 entitled "Operating Time Indicator" and CurtisBeusman's U. S. Pat. No. 3,193,763 entitled "Electrolytic CoulometricCurrent Integrating Device," both of which are incorporated herein byreference.

The device described in these patents includes a tubular body ofnonconductive material having a bore therethrough that supports twocolumns of a liquid metal such as mercury. The adjacent innermost endsof these columns are separated by a small volume of electrolyte withwhich they make conductive contact. The outermost ends of the liquidmetal columns contact conductive leads that connect the instrument tothe source of electric current that is to be measured. In accordancewith Faraday's Law, when current flows through the instrument, liquidmetal is electroplated from the anode column to the cathode columncausing the anode to decrease in length and the cathode to increase anequal amount, the change in column length being directly proportional tothe total electric charge passed through the instrument. Of course, thischange in column length also represents a change in the position of theelectrolyte along the length of the coulometer.

Readout of the total current through the instrument may be made bycomparing the length of a column against a calibrated scale. Typicalvisual readout devices are described in the above-identified CorrsinPatent and in Beusman's U.S. Pat. No. 3,343,083 entitled"Nonself-Destructive Reversible Electrochemical Coulometer." It has alsobeen found that the coulometer may be read out electrically by measuringchanges in the capacitance between the mercury columns and an electrodesurrounding the tubular body. The details of such a readout device areset forth in Edward Marwell and Curtis Beusman's U.S. Pat. No. 3,255,413entitled "Electro-Chemical Coulometer Including Differential CapacitorMeasuring Elements" and Eugene Finger's U.S. Pat. Nos. 3,704,431 and3,704,432 entitled "Coulometer Controlled Variable Frequency Generator"and "Capacitive Coulometer Improvements," respectively, all of which areincorporated herein by reference.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for reading a coulometerby sensing the temperature of the electrolyte.

I have found that when electricity is passed through a coulometer, theelectrolyte dissipates most of the electrical energy and therefore risesin temperature relative to the mercury columns. Thus, the position ofthe electrolyte and hence the integral stored in the coulometer may beread by placing one or a number of heat-sensitive devices along thelength of the coulometer. In one preferred embodiment, a pair ofthermosensitive devices are placed in thermal contact with the surfaceof a coulometer tube and spaced apart from each other. Thesethermosensitive devices have a resistance which varies as a function oftemperature. The thermoresistive devices are coupled to a circuit, suchas a conventional Wheatstone bridge, which detects the differencebetween their resistances when they are at different temperatures. Thus,if the electrolyte has moved to a position adjacent one of thethermoresistive devices, the output of the bridge will change with agiven polarity. Similarly, if the electrolyte has moved to a positionadjacent the other thermoresistive device, the output of the bridge willchange with opposite polarity. This change in the output of the bridgemay be detected using any conventional circuit, such as a differentialamplifier.

In some cases, where the temperature difference between the electrolyteand the mercury columns is not large enough to be easily detected, ithas been found desirable to pass an AC signal through the coulometer.This signal, while it does cause the electrolyte to dissipate additionalpower and thus rise to and even higher temperature, has no effect on theoverall integration process, since it has no DC component.

In accordance with an additional preferred embodiment of the invention,means other than the application of an AC current to the coulometer maybe used to cause differential temperature effects which are related tothe position of the electrolyte in the coulometer. For example, thecoulometer may be subjected to an external source of heat and the changein thermal conductivity of a localized portion of the coulometer bodywith the change in position of the electrolyte may be monitored todetermine the position of the electrolyte.

Although the coulometer may be externally heated by any of a number oftechniques, such as making use of a heated portion of a motor whose useis being monitored, the preferred embodiment contemplates the use of asimple electro-thermal source. This souce is placed adjacent thecoulometer in such a manner that a thermo-conductive path extendsthrough the coulometer body between the electro-thermal source and thethermoelectric detector. The proximity of the electrolyte to thesource-detector combination is sensed when the electrolyte advances intothe path between the source and the detector. Due to the fact that theelectrolyte is a poor conductor of heat as compared to mercury, thedetector senses a reduction in the amount of heat received from thesource.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a coulometer with a readout systemconstructed in accordance with the present invention; and

FIGS. 2-5 illustrate various coulometers using alternative readoutschemes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is illustrated a coulometer 10 which includeselectrodes 11 and 12 at opposite ends of the coulometer body. Columns ofmercury 13 and 14 are in contact with electrodes 11 and 12 respectively.Aquantity of electrolyte 15 is disposed in the gap between the twocolumns of mercury. Thus, the two columns of mercury 13 and 14 andelectrolyte 15 are in seris connection with electrodes 11 and 12. A DCsignal source 16 supplies a current proportional to the quantity to beintegrated. If desired, a metering resistor 17 may be included in serieswith signal source 16 in order to limit the DC current passing throughthe coulometer and thus make the period of integration larger.Alternatively, a resistor could be put in parallel with source 16, orany other well known metering technique could be used.

As signal source 16 passes electricity through coulometer 10, mercury isplated from the anode electrode of the coulometer to its cathodeelectrode. This causes the electrolyte 15 between mercury columns 13 and14 to move from one electrode to the other in a direction dependent uponthe polarity of signal source 16. In a given circuit, one may even beableto vary the polarity of signal source 16 and thus vary the directionof integration.

An AC source 18 is coupled via capacitor 19 to coulometer 10. AC source18 produces an AC current which passes through the coulometer causingthe electrolyte 15 to dissipate additional energy and rise intemperature. As discussed above, if enough current is produced by DCsignal source 16, it is unnecessary to add a separate AC source in orderto raise the temperature of electrolyte 15. However, in manyapplications, integration may proceed very slowly and the amount ofcurrent passing through the coulometer without the addition of the ACsource is relatively small and produces a heat change in the electrolytewhich is relatively difficult todetect. In these situations, it istherefore often advantageous to use an AC source, such as source 18, toraise the temperature of the electrolyte without affecting integration.

A pair of thermoresistive devices, such as thermistors 20-21, are placedinthermal contact with coulometer 10. These two thermistors are wiredinto a Wheatstone bridge along with fixed resistors 22 and 23, asillustrated in the figure. The bridge is provided with power by abattery 24. The output of the bridge is coupled to a differentialamplifier 25. When the electrolyte moves to a position adjacentthermistor 20 or 21, the thermistor adjacent the electrolyte will heatup and change its resistance. This causes an unbalance in the bridge anda resultant voltagechange at the input terminals of differentialamplifier 25. The polarity ofthis voltage change depends upon whetherthe electrolyte 15 is in contact with thermistor 20 or 21. The output ofdifferential amplifier 25 will vary dependent upon the polarity of thesignal at its input terminals.

Naturally, the response of amplifier 25 will depend on a number ofsystem characteristics. These characteristics include the width of thegap filledby electrolyte 15, the dimensions of the thermistor, thethermal conductivity and dimensions of the tubular body of thecoulometer, and thethermal nature of the systems' immediate environment.The transition regioncan be minimized for digital functions such as timedelay switches, cycle timers and precise end-of-integral determinationby making the distance between the thermistors large in comparison withthe gap and the dimensions of the thermistors. Conversely, thetransition region can be expanded for analog uses such as integrationand proportional control analog memory by making the distance betweenthe thermistors small in comparison with the gap and the dimensions ofthe thermistors. The shape of the transition region may also be tailoredthrough dimensional control and thermal design.

An alternative electro-thermal technique for the readout of a coulometerisillustrated in FIG. 2. In accordance with this technique, thecoulometer 30comprises a glass envelope 31 and a pair of endcaps 32.Contained within the tubular or elongated body 31 are two columns ofmercury 33 and a quantity of electrolyte 34. A pair of electrodes 35 arepositioned at opposite sides of the coulometer 30.

The operation of the coulometer illustrated in FIG. 2 is similar to thatofthe coulometer illustrated in FIG. 1 inasmuch as the position of theelectrolyte 34 is proportional to the stored integral. The position oftheelectrolyte is detected by measuring its thermal conductivityrelative to the thermal conductivity of the mercury columns. The thermalconductivity is measured using a distributed heat source 36 and aplurality of temperature sensing elements 37a-g.

Insofar as the thermal conductivity of the electrolyte 34 issignificantly less than the thermal conductivity of mercury columns 33,a thermosensitive detector adjacent the electrolyte will not receive asmuchheat as a thermosensitive detector adjacent the mercury columns.Thus, the thermal path through electrolyte 34 between heat source 36 andthermosensitive detector 37d will not conduct as much heat as the pathswhich extend through the mercury columns between the heat source andthermosensitive detectors 37a-c and 37e-g. It is thus seen thatthermosensitive detector 37d will detect a temperature smaller than thatdetected by thermosensitive detectors 37a-c and 37e-g. This differencemaybe detected by appropriate circuitry which may be used to drive anydesireddisplay for indicating the integral stored in the coulometer.

Another embodiment of a coulometer reading system using an external heatsource is illustrated in FIG. 3. This system is particularly useful forsituations where it is desired to keep the integral which one ismonitoring within a given range. The coulometer 30 used in FIG. 3 isidentical to the one used in FIG. 2 and comprises a glass envelope 31, apair of endcaps 32, two columns of mercury 33, a quantity of electrolyte34 and a pair of electrodes 35. The position of electrolyte 34 isdetectedusing a point source of heat 38 and a pair of thermosensitvedetectors 39a-b.

If the stored integral is at the desired value, electrolyte 34 isadjacent heat source 38. As a consequence of this, any heat passing fromthe heat source to thermosensitive detectors 39a-b must pass through theelectrolyte 34. Since electrolyte 34 is a relatively poor conductor ofheat, when the electrolyte is in the central position adjacent the heatsource, thermosensitive heat detectors 39a-b will have a relatively lowtemperature. If, however, the electrolyte were to migrate from thecenter position as is illustrated in FIG. 3, a relatively goodcontinuous path will exist between the heat source and one of thethermosensitive detectors. This situation is illustrated in FIG. 3wherein a relatively high quality thermoconductive path between source38 and themosensitive detector 39b results in a significant differencebetween the temperature sensed by detectors 39a-b. Thus, whenelectrolyte 34 migrates to an off-center position, as is illustrated inFIG. 3, the difference in the temperatures sensed by detectors 39a and39b will activate appropriate control circuitry to restore and maintainelectrolyte 34 in a central position.

Such a control function can be enhanced by using control circuitryresponsive to the stored integral to correct the value of that integralattwo different rates. The first of these rates would correspond tominor deviations in the value of the integral and would be relativelyslow and the second of these rates would correspond to large deviationsin the value of the integral and would be relatively fast. A coulometerparticularly suited for this sort of operation is illustrated in FIG. 4.The coulometer illustrated in FIG. 4 is substantially identical to thecoulometer illustrated in FIG. 3 with the difference that the thermaldetectors 39a-d and heat source 38 are situated on opposite sides of thecoulometer. Detectors 39b and c correspond to relatively minordeviations in the value of the integral while detectors 39a and dcorrespond to relatively large deviations from the desired value of theintegral.

It is also noted that the electro-thermal techniques of the inventionare applicable to a wide variety of coulometers. For example, as isillustrated in FIG. 5, a copper coulometer may be read using anelectro-thermal technique. The copper coulometer 40 comprises acoulometertube 41 which is made of a non-conducting material. A copperenclosure 42 closes one end of the coulometer and is coupled byconductor 43 to an external circuit. The other end of the coulometer isclosed by a copper body 44 which is coupled to an external circuit byconductor 45. The coulometer is filled with a quantity of electrolyte46. Electrolyte 46 extends from enclosure 42 through coulometer tube 41.Tube 41 also includes a column 47 of copper which is integral withcopper body 44.

When current is passed through coulometer 40 via conductors 43 and 45,copper is either removed from column 47 and plated onto the insidesurface48 of enclosure 42 or removed from the inside surface 48 andplated onto the column 47, dependent upon the direction of current flowthrough the device.

The position of the junction or interface 49 between copper column 47and electrolyte 46 is detected by monitoring the conductivity between aheat source 50 and thermosensitive detectors 51a-b. The operation ofheat source 50 and detectors 51 a-b is similar to the operation of thosedevices in the systems illustrated in FIGS. 2-4. When junction 49 is inthe centered position, the thermoconductivity of the path between heatsource 50 and detector 51a is relatively poor while thethermoconductivitybetween heat source 50 and detector 51b is of arelatively higher quality. Should junction 49 migrate substantiallyeither to one side or the other, this relationship will be altered. Suchalteration can be readily detectedby appropriate circuitry connected todetectors 51a-b.

While a preferred embodiment of the invention has been described, it isunderstood that various changes and modifications will be obvious tothoseskilled in the art. For example, the thermoresistive devices may bereplaced by thermoelectrical devices or even conventional semiconductordevices which exhibit variations in their characteristics withtemperature. It is contemplated that these changes are within the scopeofthe invention as defined by the following claims.

I claim:
 1. An integrating circuit for integrating a current,comprising:a. coulometer which includes:i. an elongated electricallynon-conducting body defining a bore; ii. a first column of liquid metaldisposed in said elongated body; iii. a second column of liquid metaldisposed in said elongated body; iv. a quantity of electrolyte disposedbetween said first column of metal and said second column of metal, saidelectrolyte defining a region which moves along the length of thecoulometer when the coulometer is subjected to a DC current; v. a firstelectrode in contact with said first column of metal; and vi. a secondelectrode in contact with said second column of metal; b. means forsubjecting said coulometer to an external source of heat; c. at leastone thermosensitive device disposed adjacent said coulometer at a firstpoint between said first and second electrodes of said coulometer; andd. circuit means coupled to said thermosensitive device for providing asignal when the temperature sensed by said thermosensitive devicechanges.
 2. An integrating circuit as in claim 1, wherein the externalsource of heat is supplied by a heat source disposed adjacent saidcoulometer at a second point between said first and second electrodes ofsaid coulometer.
 3. An integrating circuit as in claim 1, wherein saidquantity of electrolyte has a thermal conductivity different from thethermal conductivity of the metal in each of said columns.
 4. Anintegrating circuit as in claim 3, wherein said thermosensitive deviceis positioned such that a heat conduction path is formed between theheat source and said thermosensitive device, whereby the amount of heatdetected by said thermosensitive device will be affected by theinterposition of the electrolytic region between the heat source andsaid thermosensitive device and thus the position of said region can bedetected.
 5. An integrating circuit as in claim 1, wherein the metal ineach of said columns is mercury.
 6. An integrating circuit as in claim1, wherein there are at least two thermosensitive devices placed apartfrom each other and in thermal contact with said tubular non-conductingbody.
 7. An integrating circuit for integrating a current, comprising:a.a coulometer which includes:i. an elongated electrically non-conductingtube defining a bore; ii. a column of a first material disposed in oneend of said tube; iii. a column of a second material having a thermalconductivity different from the thermal conductivity of said firstmaterial, said second material being disposed in the other end of saidtube in contact with said first material and defining an interfacetherebetween, said interface moving along the length of the coulometerwhen the coulometer is subjected to a DC current; iv. a first electricalconductor in contact with said first column of material; and v. a secondelectrical conductor in contact with said second column of material; b.means for subjecting said coulometer to an external source of heat; c.at least one thermosensitive device disposed adjacent said coulometer ata first point between said first and second conductors of saidcoulometer; and d. circuit means coupled to said thermosensitive devicefor providing a signal when the temperature sensed by saidthermosensitive device changes.
 8. An integrating circuit as in claim 7,wherein the column of said first material closes one end of said tubeand a conducting enclosure of said first material closes the other endof said tube, and said second conductor contacts said second column ofmaterial via said conducting enclosure.
 9. An integrating circuit as inclaim 7, wherein at least two thermosensitive devices are positionedadjacent said coulometer at different points between said first andsecond conductors of said coulometer and in thermal contact with saidcoulometer.
 10. An integrating circuit as in claim 9, wherein circuitmeans are coupled to said thermosensitive devices for detecting theposition of said interface by monitoring the temperature at each of saidtemperature sensitive elements.
 11. An integrating circuit as in claim7, wherein said external source of heat is supplied by a heat sourcedisposed adjacent said coulometer.
 12. An integrating circuit as inclaim 7, wherein the first material is a metal and the second materialis an electrolyte.
 13. An integrating circuit as in claim 12, whereinthe metal is copper.
 14. An integrating circuit as in claim 12, whereinthe thermal conductivity of the metal is great in comparison with thethermal conductivity of the electrolyte.
 15. In a coulometer, comprisingan elongated electrically non-conducting tube, two electrodes, and afirst material and a second material within said tube in seriesconnection between said electrodes, said materials having differentthermal conductivities, said second material contacting said firstmaterial and defining an interface therebetween, said interface movingalong the length of the coulometer when the coulometer is subjected to aDC current,a method for reading said coulometer comprising the steps of:conducting heat from a heat source through a portion of the coulometerto at least one thermosensitive device; and detecting the presence ofsaid interface adjacent a point on said coulometer by monitoringtemperature at said point.
 16. In an integrating circuit whichintegrates current, a coulometer which includes a region having a firstthermal conductivity disposed between two columns of material having asecond thermal conductivity, said region moving along the length of thecoulometer as material is deposited from one of said columns to theother of said columns when the coulometer is subjected to a DC current,amethod for reading said coulometer comprising the steps of: conductingheat from a heat source through a portion of the coulometer to at leastone thermosensitive device; and detecting the proximity of said regionadjacent a point on said coulometer by monitoring temperature at saidpoint.